Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1136 AM EDT Wed May 08 2019
Valid 12Z Sat May 11 2019 - 12Z Wed May 15 2019
15 UTC Update...
Overall, changes to continuity were relatively small during this
forecast update. A blend of the 00Z ECMWF/CMC and 06Z GFS were
used as a forecast starting point during days 3-5 (Sat-Mon).
Models handle shortwave energy crossing the Great Lakes during
this time period similarly, with differences primarily confined to
amplitude. Solutions are also similar with respect to cutoff
southern stream energy skirting along the southwest U.S./Mexico
border Sat-Mon. Primary differences emerge by later in the
forecast period, and weighting of ensemble means (ECENS/GEFS) was
increased during this time frame to account for increased
uncertainty. ECMWF/CMC continue to show a slower/more amplified
shortwave crossing the Northeast Tue, with a resultant deeper
surface cyclone closer to the Mid-Atlantic/Northeast coastline
than the GFS, which is weaker aloft and more suppressed with the
surface system offshore. At this time, preferred to lean a little
more toward the more amplified solutions at 500 hPa, with a
resultant blended solution for this system very close to the CMC.
Models also show significant differences with respect to troughing
near/off the West Coast by early next week, with a high degree of
run-to-run variability lending credence to a mostly ensemble-based
forecast. Solutions were closer with southern stream energy as it
moves into the Southern Plains early next week, with similar
timing and differences primarily confined to whether a
consolidated upper low remains or whether the system begins to
more quickly shear apart with the energy disbursing across a wider
area.
Ryan
Previous Discussion (issued at 0700 UTC)
...Overview...
Expect mean troughing aloft to settle over eastern North America,
consisting of a lead amplifying trough reaching the
Plains/Mississippi Valley during the weekend and continuing
east/northeast thereafter followed by a more shallow trough whose
core should track north of the Great Lakes by the middle of next
week. The first trough will spread moisture northeastward over
the eastern U.S. while giving eastern Texas and the Lower
Mississippi Valley at least a brief reprieve from a heavy rainfall
threat that should extend through Sat. Meanwhile the pattern over
the West takes on a Rex block appearance during the weekend/early
next week as an upper ridge drifts over the northern half of the
region and an upper low slowly tracks over/near the Desert
Southwest and southern Rockies. The upper low will promote
multiple days of precipitation over southern parts of the West
with activity eventually extending into the southern Plains and
vicinity as the feature approaches and reaches the region.
Farther westward the models and ensemble means continue to show a
trough amplifying off the West Coast during Mon-Wed.
...Guidance Evaluation/Predictability Assessment...
For the full forecast domain a blend of mostly operational
guidance early followed by a trend to 60 percent 18Z GEFS/12Z
ECMWF means by day 7 Wed provided the best starting point to
reflect the most agreeable aspects of the forecast while
accounting for other cases where spread still exits. This
approach yielded a solution that maintained or only modestly
refined continuity.
For the evolution over the eastern U.S. from Sat onward, guidance
seems to be making only a gradual attempt to converge toward an
intermediate solution for the system approaching/reaching the
Great Lakes during the weekend. There are still meaningful
medium-smaller scale detail differences aloft that tend to have
fairly low predictability several days out in time. The GFS is
still on the stronger side of the spread but aside from the 00Z
run (which has an odd evolution aloft Sun-Mon) it has shown some
trends toward weakening over the Lakes with greater emphasis on
low pressure to the east.
Regarding the overall eastern trough aloft and the leading low
pressure that eventually tracks off the northeastern coast, the
ECMWF/CMC have been the most amplified aloft with surface low
pressure generally on the northwestern side of the full spread
(similarly for the extent of associated moisture). Thus far an
intermediate solution appears best as trends have not yet
displayed a pronounced shift to either side of the envelope.
Farther upstream guidance has become better defined over the past
couple days for a front that should reach the northern Plains by
day 5 Mon and continue south/east thereafter. For the time frame
involved there are typical differences in southward extent.
Consensus has held up fairly well for the upper low tracking along
the western U.S.-Mexico border and then in weaker form over the
southern Rockies/Plains. Multi-day history has generally favored
solutions in the middle to slower half of the spectrum.
For the trough amplifying off the West Coast the primary
consideration was exclusion of the CMC whose latest runs are
questionably fast to bring height falls into the Northwest. By
day 7 Wed some GFS runs become somewhat faster than consensus with
at least a portion of the trough as well. Beyond that, individual
models/ensemble members still suggest considerable uncertainty
over how energy will be distributed within the trough--favoring an
ensemble mean/blend approach.
...Weather Highlights/Threats...
The combination of waviness along a stalled front over the western
Gulf Coast/Lower Mississippi Valley, low level Gulf inflow, and
height falls aloft will extend the heavy rainfall threat over the
region into Sat. Then northeastward progression of low pressure
should bring drier weather for a time. Confidence remains lower
than desired for the axis of heaviest rainfall over the East from
Sat night onward--as far northwest as the Ohio Valley/Northeast or
as far south as the Tennessee Valley/Mid-Atlantic. Depending on
exact evolution at the surface and aloft over/off the Northeast,
highest terrain of New England could see a little snow from this
system. Farther west, expect a weakening system to bring mostly
light amounts from the northern Plains into Great Lakes during the
weekend. The next front dropping southeast from the northern
Plains may be accompanied by some rainfall by next Tue-Wed.
The upper low affecting the southern parts of the West will bring
areas of terrain-enhanced precipitation (mostly rain) from the
weekend into early next week. Expect rainfall to increase over
the southern Plains Mon-Wed as the feature approaches and reaches
the area. However there is currently poor agreement regarding the
coverage/intensity of potential showers and thunderstorms.
Other portions of the West may see scattered rainfall at times,
and then cloudiness/moisture should increase along the West Coast
states toward Tue-Wed as the upper trough approaches. Low
confidence details of the trough will determine the
coverage/intensity of any associated precipitation.
Very warm temperatures will continue over the Northwest through
the weekend with broad coverage of plus 10-20F anomalies (possibly
locally higher on Sat). There may be better potential for record
highs/warm lows in the short range period through Fri but some
potential may extend into the weekend. The approaching upper
trough will lead to a gradual moderating trend next week. On the
other hand the system affecting the southern parts of the West
will keep the Southwest into southern Plains below normal for
highs with some minus 10-20F anomalies. Eastern U.S. temperatures
will vary with system progression.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml